Get what your saying Zozo however the experiences I've had with it and others was that it doesnt grow both. Once in a tank it grows taller leaves but they drape across the surface submerged, as per all the layout images on the link but doesn't grow 'Lily pads' once submerged.
Really it's by the by, as far as the OP goes it will get bigger fella.
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Yup as said i experience the same thing with other nymphoides sp. and i still don't know which one it is, it yet didn't flower.. Smaller than the Taiwan. With lack of light it strugles and does not grow floaters. And what i find remarkable is indeed it keeps on growing a stem as you say drapping the submersed leaves along the surface.
It's this one in the indoor aquarium.

At first i thought it is a flow issue, because it's planted directly under the spraybar. But as you see there is a little flee floating runner i took from the garden and into the tank. And it also strugles to grow on even tho it simply floats.
Here is the same Nymphoides outdoor in the sun, excusively growing (also red colored) floaters

and new complete little floating plants with root development attached. (what is actualy the english botanical term for that vegitative propagation method - like a runner but than floating). Conclusion, with putting new little rooted mini floaters like this in the indoor aqaurium just floating around and refusing to grow. It must be a light issue.
The aqaurium actualy is also lit from a Skylight, it recieves a fair amount of daylight. But still aint enough during this time of year.. And indeed same as the Nymphaea depending on the light source they do not grow both forms, it is either submersed form or emersed form.
Tho the Nymphaea has transformation stages.. Also have one in this indoor tank and during the summer this nymphaea exclisively grows floaters. But nearing the fall is stops and starts growing submersed form leaves and slowly shedding it's floaters. This can take months for a floater to wear out of energy. And that time i have them both, sub en emersed for on the same plant. Sometimes i also see this in unnatural lit aqauriums, probably a tipping point confusing the plant and just enough light to grow both forms.
🙂 In my case next spring it will revert this process again and sheds submersed form and grows floaters again.
Actualy funny to see this kind of addaptions happen and change with a tank that has a dynamical natural light source. It's intersting and helps to understand some plant sp. a little beter.
🙂